1. Field of Invention
The present invention relates to the digital transmission of information; and more particularly, to a system and method for transmitting successive bursts of digital information. Although the present invention is suitable for use in many different types of communication systems, it is particularly advantageous in cellular communication systems for transmitting frames of digital information; and is herein described in connection therewith.
2. Discussion of Related Art
A cellular communication system is a mobile telephone service wherein radio coverage is divided into cells. Each cell is assigned a number of available radio frequencies. The same frequencies or channels used in one area or cell are also used for areas that are spatially separated from one another. A mobile telephone station transmits and receives control and voice communication information from a base station within the same cell. The base stations are controlled by a cellular system switching and control network that provides connection with the world wide telecommunications network. A call in progress is not interrupted as the mobile station travels from one cell location to another, since the system provides for automatic reassignment to an available channel within the other cell. Some of the assigned channels are used for control functions, such as locating a mobile station, for example, and the remaining channels for voice communication.
In order to provide superior non-interfering transmission and reception, as well as compatibility among many different mobile stations and base stations in different parts of the world and the capability of communicating with the world wide telephone network, various operational and material specifications and standards were developed to which all suppliers and users are obliged to follow.
A mobile station that operates in the digital mode is required to use an RF band which is divided into two separate 25 MHz wide segments, each consisting of 832 individual channels. The first segment contains the mobile station transmit channels and the second segment, the mobile station receive channels. The effective maximum radiated power (ERP) for various classifications of mobile stations is required to be from 0.6 watts to 4 watts depending on the classification.
Also, each channel for digitally transmitting information is required to have a frame format; that is, each channel radiates a succession of frames, each of which has a duration of forty milliseconds, and constitutes one cycle of a regularly recurring series. Each frame has six slots and each slot has 162 symbols and a duration of 6.67 milliseconds. Each symbol is made up of 2 bits of data. Each slot contains a separate burst of RF energy that is required to ramp up to peak power during a predetermined number of symbols at the beginning of each slot and ramp down to zero power during a predetermined number of symbols at the end of each slot.
It has been determined that an RF power output of a digital channel that increases or ramps up, or decreases ramps down, from a peak value, at too great a rate at the beginning and end of a slot causes spectral splattering of the transmitted signal; and an RF power output that ramps up or down at too slow a rate results in an overlapping of the slots and undesired interference between two user's slots.
It has been determined by the cellular standards committee that transmission is to be effected by ramping up to a peak voltage at the start of each burst during the first three symbols of a slot; and a ramping down of the burst voltage at the end of each burst beginning during the last three symbols of the slot.
Conventionally, a transmitter that has a digital channel and a frame format has six bursts per slot with only one mobile station using the channel at any one time. In such a system, a 6 tap spectral filter can be used that typically has coefficients sufficient to effect a peak power ramp up within the first three symbols, and a power ramp down from peak power within the last three symbols of a burst.
It has been proposed to allow multiple users to share common channels in order to increase the capacity of the system. For example, three different users may use alternate slots of each frame. The sharing of each frame tends to cause an overlap of the data of the slots, which creates noise and inferior communication.
In order to improve the spectral response of a shared channel, a finite impulse response filter having a greater number of taps is utilized. However, this advantage is offset by the slower ramp up time to peak power and the slower ramp down time from peak power for each burst. Thus, since optimum ramp up or down is not feasible with the more resolute filter, a separate ramp generator is required to be connected to the output power amplifier in order to conform to the desired standard and provide optimum ramping of the power output.